THE EVOLUTION OF STELLAR SYSTEMS 89 



present position as permanent members of our system to the 

 action of the planets. When a comet coming from outer 

 space in a parabohc orbit approaches a planet its motion is 

 either accelerated or retarded. In the latter case the parabola 

 becomes an ellipse, and the comet henceforth moves in a closed 

 path around the sun, always coming at each revolution to (or 

 near to) the point where this retardation commences. Thus 

 arise the planets' families of comets. A very large number of 

 members of Jupiter's family of comets are known ; Halley's 

 famous comet is a member of Neptune's family. In a similar 

 manner it is supposed that the asteroids and satellites have 

 attained their present positions. The whirling of the gaseous 

 matter of a spiral nebula is considered to be due to the un- 

 symmetrical meeting of two streams or to the settling down of 

 a nebula of unsymmetrical figure. From this ultimately results 

 a star surrounded by a system of planets and satellites. The 

 two opposite branches of spiral nebulae often seen on photo- 

 graphs represent the " original streams of cosmical dust which 

 are coiling up and forming spiral systems." If the streams so 

 converge that the nebulous mass becomes very concentrated, 

 the nebula may divide at its centre and give rise to a double 

 star. 



This theory of the capture of the planets and the rounding 

 of their orbits by the action of the resisting medium gives 

 results in some cases the exact opposite to those which are 

 given by the theory of tidal evolution, as investigated by Sir 

 George Darwin. 



Whilst tidal friction usually increases the major axis and 

 eccentricity of an orbit, the resisting medium as regularly 

 decreases both elements. *' In the actual physical universe 

 both causes are at work together, sometimes one influence 

 preponderating and then the other." With a large central sun 

 and small planets, as in our system, the action of the resisting 

 medium is most effective ; for systems made up of two large 

 masses, tidal friction is the predominating agency. 



There can be little doubt that these researches form a most 

 important advance in our knowledge of the genesis of our 

 system ; and though answers more or less satisfactory may be 

 found to parts of the criticism of Laplace's famous hypothesis, 

 yet we may fairly say that, if not completely disproved, it has 

 been very seriously undermined. 



